While the task of drilling planning and well path/well trajectory identifications is primarily an engineering function, a critical objective of drilling planning is to maximize the output of the oil/gas extraction from given reservoirs. Understanding of the reservoir properties as well as geological constraints, such as potential hazard avoidance, is vital to the success of a drilling program.
In a currently typical work flow of a drilling planning session, for each planned well, a potential drill center location (on the surface) and a set of one or more (subsurface) target locations are selected based on the reservoir properties. Geoscientists and engineers can reposition the targets and/or relocate the drill center location to obtain a satisfactory well trajectory while meet most of, if not all, the engineering and geological constraints in an interactive planning session. In this current practice, the targeted locations represented by points in 3D space would have been pre-determined based on the geological/reservoir models for reservoir productivity by geologists and reservoir engineers. Often, an optimization algorithm is then used to find the optimal drill center location for those pre-determined target locations based on engineering and drilling constraints. How this drilling planning is currently done is discussed further in the following paragraphs.
The oil field planning involves optimization of a wide variety of parameters including drill center location(s), drill center/slot design, reservoir target location(s), well trajectory and potential hazard avoidance while maximizing stability and cost-effectiveness given the stratigraphic properties with wide variety (often conflicted) constraints. Current field/drill center design practices are often sequential and can be inefficient, for example:
1. Geoscientist selects potential targets based on geologic interpretation and understanding of reservoir properties.
2. Multiple well trajectories are designed and given to the drilling engineer for more detailed well design and analysis.
3. The drill center locations are selected or modified based on the results of the well design and analysis step.
4. Changes to the target location(s), number of targets, or basic trajectory parameters are made during the iterative steps by geologists and drilling engineers; depending on the complexity of the well path and geology, the final drill center locations and well trajectory may take many such iterations and several weeks/months of calendar time.
Several factors affect the selection of well drill center locations and their configuration since it is an integral part of an optimal capital investment plan including fields, reservoirs, drilling centers, wells, etc. See, for example, Udoh et al., “Applications of Strategic Optimization Techniques to Development and Management of Oil and Gas Resources,” 27th SPE meeting, (2003). Optimization technology in the current state of the art places primary focus on how to determine and optimize each component. For example, U.S. Pat. No. 6,549,879 to Cullick et al. discloses a two-stage method for determining well locations in a 3D reservoir model. Well location and path is determined while satisfying various constraints including: minimum inter-well spacing, maximum well length, angular limits for deviated completions and minimum distance from reservoir and fluid boundaries. In their paper titled “Horizontal Well Path Planning and Correction Using Optimization Techniques” (J. of Energy Resources Technology 123, 187-193 (2003)), McCann et al. present a procedure that uses nonlinear optimization theory to plan 3D well paths and path correction while drilling. This process focuses primarily on engineering criteria for well trajectory such as minimum length, torque and drag as well as some other user imposed constraints. In another paper, “Well Design Optimization: Implementation in GOCAD” (22nd Gocad Meeting, June, 2002), Mugerin et al. present an integrated well planning that includes geological and engineering constraints for target selection and path generation. U.S. Pat. No. 7,460,957 to Prange et al. presents a method that automatically designs a multi-well development plan given a set of previously interpreted subsurface targets.
From the above-described practices and arts, one can see well path planning often involves geological and/or engineering constraints to derive a set of optimal well paths. Significant challenges remain such as integrating optimal well path constraints with finding optimal drill center locations, since the conflicting objectives of well targets, well paths and/or drill center locations may complicate the optimization process which would lead to sub-optimal solutions. Furthermore, as stated by Prange et al., the proposed multi-well trajectories optimization that relies on a set of pre-selected fixed targets could further limit the selection of optimal drill center configuration since the constraints on the drillable well trajectories to multiple fixed targets would add extra complexity to the overall optimization processes and may not lead to an optimum solution.